Silicon carbide (hereinafter also referred to as SiC) is expected as a next-generation power semiconductor device. SiC has excellent physical properties compared with Si. That is, SiC has a bandgap three times that of Si, breakdown field strength about ten times that of Si, and thermal conductivity about three times that of Si. An extremely-low-loss power semiconductor device that can be operated at a high temperature can be implemented using the characteristics of SiC.
There are various high-voltage semiconductor devices in which the characteristics of SiC are utilized. For example, there is well known a Double Implantation MOSFET (hereinafter referred to as DIMOSFET) in which a p well and a source region are formed by ion implantation.
The DIMOSFET is easily produced because of use of a planar process in which a channel can accurately be formed by an ion implantation method. Because a gate is driven by voltage control, a power of a driving circuit can be reduced, and the DIMOSFET is suitable to a parallel operation.
However, in the device made of SiC, because channel mobility is degraded by an interface state of a MOS interface, a channel resistance (on resistance) of the MOSFET is much higher than that of the device made of Si. A method for setting a channel length to 1 μm or less has been proposed in order to solve the problem.
When the channel length is shortened, a leakage current is increased between a source and a drain at a high temperature of 250° C. or more, whereby a breakdown voltage of the device decrease. Therefore, a balance between a high-temperature operation and a low resistance (low on resistance) of the channel is hardly achieved.